Wireless communication systems utilize transceiver "nodes" (e.g., cellular base stations or satellites) to transmit information to and receive information from communication units (CUs) (e.g., mobile subscriber units). Typically, a node provides communication channels within one or more regions called "cells". The geographical boundaries defining a cell can be fixed or moving. When a CU is located within the boundaries of a cell which has a sufficient link margin, the CU can exchange messages and information with the node. The link margin is a function of the power level of a signal which a CU receives and is used as a measure of the link quality.
In a communication system where nodes and CUs move with respect to each other, hand offs of a CU from one node to another become necessary in order to provide continuous service to the CU. For example, the user of a mobile CU might travel from one cell to another, making a hand off necessary. Alternatively, the cells provided by the nodes might move across the surface of the earth. Cell movement would occur, for example, in a system which uses non-geosynchronous satellites having satellite-fixed cells (i.e., cells which are fixed relative to the satellite, but move relative to the surface of the earth).
In order to hand off a CU from one cell to another without interruption of the communication link, the physical areas which define a system's cells must be contiguous or must overlap. In prior art systems, a node provides multiple cells which can partially or completely overlap each other. In systems utilizing multiple nodes, each node provides cells which partially or completely overlap with cells from other nodes. When a communication unit is located within an overlapping cell region, multiple cells theoretically are available to the unit and the CU could be handed off to any of the available cells.
In prior art systems, a cellular base station controls hand offs based on signal power measurements received from other base stations which are interspersed throughout the area which the cellular system typically services. The prior-art technique has several drawbacks and is impractical for modern wireless communication systems which service vast geographical areas. One drawback to the prior-art technique is that, in most cases, the base stations which perform the power measurements are not co-located with the CUs. Thus, the power measurements made by the base stations are typically inaccurate compared to the actual signal power at the CU locations. The controlling cellular base station, thus, is forced to control hand offs based on that inaccurate information. Another drawback to the prior-art technique is that the base station measurements typically are infrequently made. Thus, the controlling cellular base station is forced to use power measurements which are not current. This also leads to inaccuracies in controlling hand offs.
The prior-art technique would be impractical for use with a wireless communication system which uses satellites as transceiver nodes. One reason is that the inaccuracies associated with the prior-art technique would be compounded in a non-geosynchronous satellite system. Unlike ground-based systems, satellite movement causes cell link margins across the surface of the service area constantly to be changing. Thus, infrequently made power measurements would significantly affect the accuracy of the base station calculations.
Typically, satellite systems provide communication services to large areas (e.g., the surface of the entire earth). Thus, another reason that the prior-art technique would be impractical for satellite systems is that the expense and difficulty associated with providing dispersed power measurement facilities (e.g., within cellular base stations) in a satellite communication system would be prohibitive.
What is needed is a method and apparatus to enable a communication system accurately to predict link margins within the system. Further needed is a method and apparatus for specific subsystems within the communication system to enhance service in areas which have poor link margins based on link margin predictions.